Alternator-starter systems may be used in vehicles to provide engine cranking for engine starts and also to generate power for vehicle electrical auxiliary loads (also known as hotel loads). In some engines, gas consumption and emissions are reduced by “on-and-off” operation, that is, by turning the engine on and off based on emission and gas consumption. Such engines may need repeated starting, for example, during urban stop-and-go operation. Starting torque for an engine typically needs to be much higher, although of shorter duration, than torque for driving the vehicle. On the other hand, generating power needs to be provided over a wide engine operating range, from engine idle speed to engine maximum speed.
Several alternator-starter systems are commercially available. Such systems may be based, for example, on modified conventional Lundell alternators, induction machines, switched reluctance machines, or strong-flux IPM machines. Such systems can have drawbacks. For example, some Lundell-based alternators can suffer from poor efficiency. Induction machine-based systems may provide poor torque density and may have difficulty providing generation power over a wide speed range. Switched reluctance systems also can provide poor torque density and operational efficiency and can have thermal drawbacks. While some permanent magnet systems offer good torque density, high spin loss associated with high magnet flux can detract from their operational efficiency at high speeds. If electrical faults occur, the reliability of such systems also can be reduced.